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Full text of "Text Book Of Mechanical Engineering"

870                           Appendix II.

lubricant alters with the temperature, every oil having a best
condition, but the results of flat friction at linear velocities from
400 to 1600 ft. per m, shew a constant /* of '23 at 50 Ibs. per
sq. in. ; for cast-iron. surfaces. The apparatus is shewn in Fig. 831,
where A is a rotating disc, and B B two rubbing surfaces (shewn
in detail on the right) pressed upon it by the load c. The
turning effort of D is resisted by string E, and Fn measured by
spring-balance or scale-pan weights.

Beauchamp Tower's journal experiments for the Inst. of
Mech. E. were made at high speeds with heavy loads, and the
projected area Id adopted for measurement of unit pressures,
d being width of journal embraced, and / its length. The
nominal f was thus conventional, the real p often rising to twice
its amount. The load was carried on a knife-edge K, Fig. 832
and c K would assume some position c L. Then

Moment of friction = Moment of weight

The movement was multiplied by levers A and B, but still
being small, was measured by weights in a scale-pan D sufficient
to bring B back to zero. The starting stickiness was overcome
by a small weight E, -and the lever thus kept horizontal for zero
load, the amount being decided by revolving in each direction
and taking the average. Briefly, total friction was found con-
stant at all loads, and p. varied as i ~ P. Oil-bath lubrication
proved best, sponge lubrication some four times as resistant,
and siphons gave still worse results. Temperature materially
altered the friction; thus by lowering from 300 to 120 Fahr.
the friction with lard oil was as 1:3. By means of pressure
gauges it was shewn to be useless to introduce the lubricant
where pressure was greatest, for the latter often rose to 200 Ibs.
per sq, inch and forced out the oil. Oil should therefore enter
at the top of shafts and the bottom of axles, and grooves be cut
to assist it. (See Third Prefaced)

With collar bearings (Inst. Mech. E. experiments) the friction
varied more nearly with the pressure, the apparatus being shewn
in Fig. 832*7. A ring c supported on rollers jj, was pressed